1. Field of the Invention
This invention relates to a method and apparatus for transforming a color image. This invention particularly relates to an image transforming method and apparatus, wherein transformed values are calculated previously by carrying out operations with respect to a limited number of values (colors), and a transformed value corresponding to an arbitrary picture element value is calculated by carrying out an interpolating operation on the previously calculated transformed values.
2. Description of the Prior Art
Color transformation has heretofore been carried out as one kind of digital image processing. For example, in the cases of digital processing of color photographs, if a digital image having been processed for the formation of a print is displayed directly on a monitor, the displayed image will not necessarily look in appropriate colors. Therefore, it is desired that color transformation with a predetermined transformation formula is carried out on the digital image, and thereafter the image obtained from the color transformation is displayed on the monitor.
In the color transformation of a color image, the picture element value of each picture element may be transformed literally by carrying out an operation with a transformation formula. However, such a technique has the problems in that, in cases where the relation formula is a complicated one, the time required to carry out the operation cannot be kept short. Therefore, ordinarily, in systems in which the processing should be carried out quickly as in digital processing of photographs, a technique referred to as the direct mapping process is employed.
In the direct mapping process, transformed values are calculated previously by carrying out accurate operations with respect to a limited number of colors, and the information representing the calculated transformed values is stored in a memory. When an actually inputted value is to be transformed, instead of the accurately operation being carried out, an interpolating operation is carried out by using the previously calculated transformed values, and a transformed value corresponding to the actually inputted value is thereby presumed. Specifically, a multi-dimensional color space is divided into a plurality of blocks, and transformed values are calculated previously by carrying out accurate operations with transformation formulas with respect to coordinates of vertexes of each block. Thereafter, a transformed value corresponding to an arbitrary point is interpolated from the transformed values of the coordinates of the vertexes.
When the interpolating operation is to be carried out, eight vertexes surrounding the inputted picture element value are specified, and the information representing the transformed values of the coordinates of the eight vertexes is read from the memory. The eight vertexes are weighted in accordance with the position of the picture element value in the block. Factors, which represent the weights of the respective vertexes, and the corresponding transformed values, which have been read from the memory, are multiplied by each other, and the thus obtained products are added together. In this manner, the interpolated value is obtained. The interpolating operation is constituted of the simple operations, i.e. the multiplications and the additions. Accordingly, with the interpolating operation, the processing can be carried out more quickly than with the technique in which the operation with a transformation formula is carried out.
In the technique described above, eight times of multiplications and seven times of additions (i.e., the adding of eight values) are necessarily carried out for one time of interpolating operation. However, in cases where the inputted picture element value is the one located at a point on a surface or a line of the block, the weight factors of the vertexes other than the vertexes, which constitute the surface or the line of the block, become 0. Therefore, in such cases, actually, it is unnecessary for the multiplications to be carried out with respect to the weight factors of the vertexes other than the vertexes constituting the surface or the line of the block.
Also, in the technique described above, the weight factors are calculated each time a picture element value is inputted. However, actually, in cases where the relative position in a block is the same, the weight factors of the vertexes do not vary. Therefore, the technique described above has the drawbacks in that the weight factors are calculated many times with respect to the same pattern.